Magnetic rotation sensor for rotary shaft

ABSTRACT

This invention relates to a rotation sensor which detects the magnetic field which changes with the rotation of a rotary shaft, thereby to detect the amount of rotation of the rotary shaft. The rotation sensor of the present invention has the boundaries between magnetized regions set with high precision. The boundaries forms magnetic fields stably, whereby the detection of rotation is performed with high accuracy.

BACKGROUND OF THE INVENTION

This invention relates to a rotation sensor which detects the magneticfield which changes with the rotation of a rotary shaft, thereby todetect the amount of rotation of the rotary shaft.

The principle of detection with a rotation sensor of this type is asfollows: A magnet, which is so magnetized that N and S poles areprovided alternately as viewed in the circumferential direction, issecured to a rotary shaft so that it is rotated together with thelatter. Under this condition, a magnetic sensor is used to detect themagnetic field of the magnet which changes with rotation of the rotaryshaft, thereby to detect the station of rotation of the rotary shaft.That is, since the N and S poles are provided alternately as viewed inthe circumferential direction, as the magnet rotates the direction ofthe magnetic field changes from circumferential direction to radialdirection or vice versa at each of the boundaries between the N and Spoles. This change is detected for the state of rotation of the rotaryshaft.

In this case, the accuracy of detection for the state of rotation; thatis, the resolving power can be increased by increasing the number ofchanges in magnetic field per revolution; that is, by increasing thenumber of magnetic poles formed in the magnet in the circumferentialdirection. In general, increasing the number of magnetic poles in amagnet will decrease the magnetic field strength per magnetic pole. Thisaffects the sensitivity of detection of the magnetic sensor, andtherefore the increasing of the number of magnetic poles makes itdifficult to improve the accuracy of detection.

In order to eliminate this difficulty, the following method has beenproposed in the art. As shown in FIG. 4, a magnet 1 is formed in such amanner that N and S poles appear alternately in a radial direction. Morespecifically, in the magnet 1, N and S poles are provided alternatelyboth in the circumferential direction and in the radial direction, sothat, in the detecting surface 2, the magnetic flux extends radially aswell as circumferentially (as indicated by the arrows In FIG. 4). Thisincreases the region where parallel magnetic fields are obtained, and,therefore, with the same magnetization, the magnetic field formed ishigher. Hence, although the number of magnetic poles is increased, thechange in the direction of magnetic field can be satisfactorily detectedwith the magnetic sensor; that is, the accuracy of detection of themagnetic sensor can be improved.

In the above-described conventional rotation sensor, the magnetizationof the magnet 1 is carried out as follows: As shown in FIG. 5, aU-shaped magnetizing yoke 3 is employed to form N and S poles in themagnetizing surface 2 of the magnet alternately in a radial direction.However, since the surface 2 is small, it is considerably difficult toform N and S poles alternately therein with high accuracy, andaccordingly it is difficult to accurately locate the neutral line Mbetween N and S poles. Hence, when the magnet 1 is rotated, the changein magnetic field detected by the magnetic sensor 4 is not uniform.Thus, the output of the magnetic sensor is unstable, and accordingly theaccuracy of detection is lowered as much.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the invention is to provide arotation sensor in which the boundaries between magnetized regions areset with high precision to form magnetic fields stably, whereby thedetection of rotation is performed with high accuracy.

The foregoing object of the invention has been achieved by the provisionof a rotation sensor which, according to the invention, comprises: amagnetic substance in the form of a hollow cylinder or in the form of asolid cylinder which rotates together with a rotary shaft; an annularmagnet having an end face which is magnetized in such a manner that Nand S poles appear alternately as viewed in the circumferentialdirection thereof, the annular magnet being fixedly mounted on themagnetic substance in such a manner that the end face thereof is flushwith the end face of the magnetic substance; and a magnetic sensor fordetecting the magnetic flux which changes with rotation of the rotaryshaft with the end faces of the magnetic substance and annular magnet asa detecting surface.

In the rotation sensor of the invention, N and S poles are formed in theend face of the magnet alternately in the circumferential direction.When the magnet is mounted on the magnetic substance, the magnetic fluxof the magnet runs in the magnetic substance, thus forming magneticpaths therein. As a result, magnetic poles are formed in the end face ofthe magnetic substance. Hence, in the rotation sensor of the invention,the neutral lines between N and S poles are more clearly with mechanicalaccuracy than in the conventional rotation sensor in which N and S polesare formed alternately in a radial direction. Accordingly, the change inmagnetic field is stable when detected by the magnetic sensor. Thus,with the rotation sensor of the invention, the state of rotation of therotary shaft can be detected with high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are a perspective view and a vertical sectional viewshowing the whole arrangement of the rotation sensor of the presentinvention;

FIG. 3 is an explanatory diagram for a description of the magnetizationof a magnet of the present invention;

FIG. 4 is a perspective view of a conventional rotation sensor; and

FIG. 5 is a diagram for a description of the magnetization of a magnetof the conventional rotation sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of a rotation sensor, which constitutes one embodiment ofthis invention, will be described with reference to FIGS. 1 through 3.

First, the whole arrangement of the rotary sensor will be described. Asshown in FIG. 1, a rotary shaft 11, which is a magnetic substance, iscylindrical, and has an end portion lla larger in diameter than theother portion. The rotary shaft 11 is engaged with an annular magnet 12.The annular magnet 12 is so magnetized that N and S magnetic polesappear alternately in the circumferential direction. More specifically,as shown in FIG. 3, a number of magnetic poles are formed in themagnetizing surface 12a of the magnet in such a manner that they arearranged in the circumferential direction, and a single magnetic pole isprovided as viewed in a radial direction. A magnetizing yoke 13 is setabove the magnetizing surface 12a. Hence, not only the magnetic polesare formed in the magnetizing surface 12a as described above, but alsodifferent magnetic poles are formed in the opposite surface which areopposite in polarity to those formed in the magnetizing surface 12a.That is, in the annular magnet 12, the magnetic poles are opposite toeach other as viewed in the axial direction (cf. FIG. 1 too). Theannular magnet 12 thus formed is fixedly mounted on the rotary shaft 11in such a manner that, as shown in FIG. 2, the inner cylindrical wall ofthe annular magnet 12 is in contact with the outer cylindrical wall ofthe end portion lla a of the rotary shaft 11, and the end face of therotary shaft 11 is flush with the magnetizing surface 12a of the annularmagnet 12. A magnetic sensor made up of a magnetic resistance element ora Hall element for magnetic detection, is positioned above the boundarybetween the magnetizing surface 12a of the magnet 12 and the end face ofthe end portion lla of the rotary shaft 11.

When the annular magnet 12 is fixedly mounted on the rotary shaft 11 aswas described above, the lines of magnetic force of each of the magneticpoles in the magnet 12, for instance the lines of magnetic force of eachN pole run through the upper end portion lla of the rotary shaft 11,which is a magnetic substance, to the respective S pole formed in thelower portion of the magnet 12 as indicated by the arrows in FIG. 2.That is, the lines of magnetic force thus run forms a magnetic path inthe upper end portion lla of the rotary shaft 11. As a result, in theupper end portion, an S pole is formed near the N pole. Thus, in theupper end portion 11, N and S poles are formed alternately in thecircumferential direction. That is, in the assembly of the upper endportion 11 and the annular magnet 12, N and S poles appear alternatelyboth in the circumferential direction and in the radial direction. Inthis case, the boundary between the magnet 12 and the rotary shaft 11 ismechanically determined; that is, in the invention, unlike theconventional art, the accuracy of magnetization is not problematic.Thus, the boundaries between the magnetic poles are clearly determined.

As was described above, in the magnetizing surface 12a, a single pole isformed as viewed in a radial direction. Hence, the magnetizing yoke 13can be set over the whole magnetizing surface 12 a, and themagnetization can be increased accordingly. Therefore, although a numberof magnetic poles are provided as viewed in the circumferentialdirection, they will not lower the accuracy in detection of the magneticsensor 14.

In the above-described embodiment, the rotary shaft 11 of iron isemployed as the magnetic substance; however, the invention is notlimited thereto or thereby. That is, for instance a magnetic substanceprovided separately may be connected to the rotary shaft. In theabove-described embodiment, the magnetic substance is in the form of ahollow cylinder; however, the same effect can be obtained by employing amagnetic substance in the form of a solid cylinder.

As was described above, in the rotation sensor according to theinvention, the magnet in which a number of magnetic poles are formed asviewed in the circumferential direction is mounted on the magneticsubstance, so that the lines of magnetic force run through the magneticsubstance, forming magnetic paths therein. This is equivalent to thefact that N and S poles are formed in the magnetic substance. Inaddition, the boundaries between the magnetic poles are located at theboundary between the magnetic substance and the magnet which ismechanically determined. Accordingly, the rotation sensor is free fromthe difficulty that the detection signal of the magnetic sensor isunstable; that is, it can positively detect the state of rotation withhigh accuracy. This effect should be highly appreciated.

What is claimed is:
 1. A rotation sensor comprising:a rotary shafthaving a hollow magnetic end portion; an annular magnet having an endface which is magnetized in such a manner that N and S poles appearalternatively in a circumferential direction thereof, said annularmagnet being fixedly mounted on said magnetic end portion in such amanner that said end face thereof is flush with said end portion of saidrotary shaft; and magnetic sensor means for detecting a magnetic flux,changing in accordance with the rotation of said rotary shaft, with adetecting surface formed by said end portion of said rotary shaft andsaid end face of said annular magnet.
 2. A rotation sensor as claimed inclaim 1, wherein said magnetic sensor means comprises a magneticresistance element.
 3. A rotation sensor as claimed in claim 1, whereinsaid magnetic sensor means comprises a Hall element.
 4. A rotationsensor as claimed in claim 1, wherein said magnetic sensor means ispositioned above a boundary between said magnetic end portion and saidannular magnet.
 5. A rotation sensor as claimed in claim 1, wherein saidmagnetic end portion has a larger diameter than said rotary shaft.